Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-user variation of Orthogonal Frequency Division Multiplexing (OFDM) which has emerged as a key technology to improve efficiency in advanced infrastructure-based wireless networks. OFDM is a modulation method that divides a channel into multiple narrow orthogonal bands that are spaced apart so as to avoid interference with one another. OFDMA combines OFDM on the physical layer with Frequency and Time Division Multiple Access on the media access control (MAC) layer, allowing the assignment of different sub-channels to different nodes in order to support Point to Multipoint operation. Sub-channels may consist of adjacent or non-adjacent subcarriers.
In OFDMA systems, the allocation of two dimensional (time and frequency) slots in the uplink and downlink direction is determined by an allocation map transmitted periodically by the base station to the remote stations. The allocation is done within the sub-channels available to each base station. Cellular wireless communication systems may employ frequency reuse, which generally refers to the process of using the same radio frequencies for communications for geographic areas that are separated by sufficient distance so as to cause no or minimal interference with each other (e.g., interference below a threshold level). The frequency re-use scheme is characterized by nomenclature (N, S, K), where N=Cells per cluster, S=Sectors per cell, and K=Number of sub-channel groups.
The system may employ Time Division Duplexing (TDD) with programmable downlink to uplink symbol ratio and programmable gaps. Each TDD frame includes a downlink sub-frame (DLSF), an uplink sub-frame (ULSF), a transmit to receive gap (TTG), and a receive to transmit gap (RTG). The TDD frame may be GPS synchronized, and its configuration may be identical across all sectors. Generally, the base station (BS) allocates distinct slots in the DLSF and ULSF in each TDD frame to satisfy the instantaneous bandwidth required for communication between the BS and each of the remote stations.
Each base station may allocate slots subject to the air interface resources available to its sector. The downlink MAP (DL-MAP) is a broadcast message transmitted in the downlink (DL) direction to transmit the map of the DL allocations to all remote stations. The basic element of the DL-MAP is an information element (IE) called DL MAP IE that specifies allocations to remote stations within a particular frame. The uplink MAP (UL-MAP) may include the UL MAP IE defining uplink (UL) allocations for all UL bursts transmitted by remote stations.